When using profile measuring interference microscopy to measure the phase of a heterogeneous test surface comprised of two or more dissimilar materials, at least one of which has a complex index of refraction, measurement errors result because the amount of phase change on reflection varies with each individual material forming the test surface. Materials with complex indices of refraction produce a phase change on reflection that, in general, varies from material to material as well as from a dielectric's phase change on reflection which is either 0 or .pi.. For example, conventional profile measuring interferometric techniques cannot distinguish, on the one hand, a phase difference between two points of different height on a test surface and, on the other, a phase difference between two points on a test surface of the same height but composed of dissimilar or composite materials with at least one having a complex index of refraction.
Use of the technique of profile measuring interferometry with an interference microscope to measure the profile of a test surface is known. Such use is described, for example, in Biegen, J. F. and Smythe, R. A., "High-Resolution Profile Measuring Laser Interferometric Microscope for Engineering Surface Metrology", presented at the Fourth International Conference on Metrology and Properties of Engineering Surfaces at the National Bureau of Standards, Washington, D.C., Apr. 13-15, 1988. As applied to interference microscopes, the profile measuring interferometric technique provides effective and accurate test surface profile measurements so long as the test surface is homogeneous, i.e. comprised of a singular material having either a complex or a non-complex index of refraction. The phase change on reflection from a homogeneous test surface is constant as a function of field position and, therefore, does not affect the accuracy of the test surface profile measurement. When, however, the test surface to be profiled is heterogeneous and the phase change on reflection accordingly varies significantly as a function of field position, the phase map produced by profile measuring interferometry no longer represents an accurate geometrical profile of the test surface. In conventional profile measuring interferometry there is no way to extract the phase change on reflection component induced by reflection of the illumination beam off the test surface from the total phase that is measured. This has been a fundamental limitation in the utility and practice of conventional profile measuring interferometry.
Also known is the technique of using values of n and k--the real and imaginary parts of a material's complex index of refraction--previously measured using instrumentation other than profile measuring interferometers, to correct subsequent profile measuring interferometry measurements. This prior art technique, however, has serious limitations. The n and k measurements are usually made on representative materials, rather than on the actual test surface, and even minor differences in material composition between the representative material and the test surface material can introduce large errors in the phase correction. Moreover, when correcting phase measurements carried out with a moderate to high numerical aperture microscope interferometer objective, both the quantities n and k and a measure of the average illumination beam angle of incidence to the test surface are needed. The average illumination beam angle, which is a function of the numerical aperture and of the illumination beam intensity distribution at the entrance pupil of the microscope interferometer objective, can only be found through empirical means with this technique and, as such, introduces a source of potentially significant measurement error in the test surface profile.
Previous techniques for the direct measurement of phase change on reflection, as for example described in J. Bennett, "Precise Method for Measuring the Absolute Phase Change on Reflection", 54 J. Opt. Soc. Am. 612-24 (1964), are difficult, time consuming, limited in use to transparent films and produce results independent of the actual test surface so that even if the measurement results are correct there is no certainty that the result actually represents the material on the test surface.
A method of directly measuring the phase change on reflection is discussed in "Measurement of Transducers on Thin Film Sliders for Rigid Disk Drives", presented at the International Disk Conference in Tokyo, Japan, April, 1992. That article describes a time consuming, two-step process which consists of first measuring the test surface with a profile measuring interference microscope, overcoating the test surface with a homogeneous opaque material, and then remeasuring the test surface again with a profile measuring interference microscope. The two profiles thus obtained are subtracted one from the other, the difference between the profiles being the phase change on reflection.
Other profile measuring interference microscope techniques that determine the axial position of maximum fringe contrast for profiling the test surface emphasize the use of broad spectral bandwidth (i.e. white light) as the preferred illumination. See U.S. Pat. No. 4,340,306 to Balasubramanian; M. Davidson et al., "An Application of Interference Microscopy to Integrated Circuit Inspection and Metrology", 775 SPIE 233-247 (1987); G. S. Kino and S. T. Chim, "Mirau Correlation Microscope", 29 Applied Optics 3775-83 (1990); and B. S. Lee and T. C. Strand, "Profilometry With a Coherence Scanning Microscope", 29 Applied Optics 3784- 88 (1990). A broad illumination spectral bandwidth reduces crosstalk between vertically adjacent features, permitting depth slicing as in confocal scanning microscopy. A broad bandwidth also allows for a theoretically unlimited test surface feature measurement range with high vertical resolution. And with a large illumination spectral bandwidth, the axial interference region is small and independent of the objective magnification or numerical aperture so that vertical resolution is constant across the magnification range and is not a function of the objective depth of focus, as it is in confocal scanning microscopy. One disadvantage of broad illumination spectral bandwidth is that the axial position of maximum interference contrast will shift in axial position as a function of the material properties of the test surface. The phase of the interference at this axial position cannot be related to the phase change on reflection without a priori information on the material properties of the test surface. This renders the technique as inaccurate as conventional profile measuring interferometry when measuring heterogeneous test surfaces comprised of two or more dissimilar materials with at least one having a complex index of refraction.
In contrast, the herein disclosed method and apparatus of the invention extend and improve the technique of conventional profile measuring interference microscopy by additionally measuring the phase change on reflection from the surface of metals, semi-metals, and dielectrics being profiled, and thereby correct a systematic measurement error that occurs when utilizing conventional profile measuring interference microscopy.